Ink jet printers are gaining wide acceptability as advances in technology make their use more practical and economical. The printers can operate at extremely high speeds and do not require mechanical displacement to enter information onto a record medium.
In general, an ink jet printer operates by issuing a stream of ink droplets from the nozzle of a droplet generator; the droplets may be issued periodically or aperiodically depending on the particular application. In aperiodic applications, the printer may have a print head made up of an ordered array or matrix of nozzles. Alphanumeric and other type characters can be formed by activating a selected pattern of nozzles to represent a character.
The droplet generator is a fundamental component in an ink jet printer. It must reliably issue an ink droplet of uniform measure at a precise velocity despite variations in ink temperature and viscosity. One form of droplet generator is the piezoelectric capillary injector. Basically, this generator comprises a body having formed within it a cell or chamber filled with ink. The ink chamber is fed by capillary action through a port in the body. One wall of the cell is closed by a diaphragm. Opposite the diaphragm is an aperture that serves as a nozzle. The diaphragm is bonded to an axially polarized piezoelectric crystal which experiences axial expansion and radial contraction when electrically stimulated. The motion of the crystal causes the diaphragm to flex. This creates a pressure perturbation in the ink chamber forcing an ink droplet to be issued from the nozzle.
The basic development of the piezoelectric capillary injector is discussed in "The Piezoelectric Capillary Injector -- A New Hydrodynamic Method For Dot Pattern Generation", by Erik Stemme and Stig-Goran Larson, IEEE Transactions on Electron Devices, January 1973, pp 14-19. The experimental model disclosed therein is basic to the art, but is found upon study to have two major drawbacks which are discussed as follows.
First, the Stemme-Stig Goran device is difficult to manufacture due to the complexity of its design. It has a central, inner liquid cell formed between the front and rear faces of the body; a thin internal liquid layer formed parallel and proximate to the front face of the body that supports ink under capillary forces; a first aperture connecting the inner liquid cell and the liquid layer; and a second aperture that connects the liquid layer and the outside and serves as a nozzle. The device requires that the first and second apertures be coaxially aligned, although the apertures are each only 40 .mu.m in diameter and are formed in adjunct body members. This is an extremely difficult criterion to meet in production models.
Secondly, the Stemme-Stig Goran device discloses no means to damp oscillatory excursions of the piezoelectric crystal. These undesirable oscillations can cause secondary issuance of droplets. Moreover, crystal oscillation limits printing speed by requiring that the excursions settle down before another print signal is applied to the crystal.
These limitations of the prior art have been an inducement for the design of an ink droplet generator that is of reduced complexity but of improved performance capability. The present invention addresses this objective.